The present invention relates generally to Q-switched lasers, and in particular to Q-switched lasers with frequency conversion in the laser cavity, and more particularly to methods for operating such lasers.
The application of Q-switching techniques to lasers has made it possible to produce short pulses with high peak-powers. Many standard Q-switched lasers are capable of producing pulses with a duration on the order of a few cavity decay times (i.e., from a few nanoseconds to many tens of nanoseconds) and peak powers from the kilowatt to the megawatt range.
In lasers without Q-switches and in which the lasing medium is continuously pumped, the population inversion (i.e., the proportion of lasant atoms or molecules in the high energy state and ready to participate in stimulated emission) is fixed at a threshold value when oscillation is steady. Even under pulsed operating conditions, particularly at high repetition rates, the population inversion exceeds the threshold value by only a relatively small amount due to the onset of stimulated emission. Q-switching techniques employ a Q-switch positioned inside the laser cavity to modulate the laser cavity loss. When the Q-switch is on cavity loss is very high and laser action is prevented. Consequently, as the lasing medium is pumped the population inversion builds up to levels far exceeding the threshold population holding when the Q-switch is absent. Now, when the Q-switch is suddenly turned off, the cavity loss decreases rapidly and the laser suddenly has a gain that greatly exceeds loss. As a result, the energy stored in the lasing medium is released in the form of a short and intense pulse.
Various types of Q-switches employing different principles have been described in the prior art. In general, these Q-switches fall into two groups: active Q-switches and passive Q-switches. Active Q-switches require external control to turn them on and off. For the most part, active Q-switches either employ mechanical elements (e.g., mechanical shutters, rotating prisms, etc.) or elements relying on the electro-optic or acousto-optic effects. Passive Q-switches typically rely on an optical nonlinearity of the element used (e.g., a saturable absorber). For more information on Q-switches the reader is referred to Orazio Svelto, xe2x80x9cPrinciples of Laser Opticsxe2x80x9d, Plenum Press, (translated by David C. Hanna), 1998, pp. 313-319.
FIG. 1 shows a typical prior art Q-switched laser 10 with an active Q-switch 12 controlled by a Q-switch control 14. A lasing medium 16 of laser 10 is pumped by a pump source 18 such as a bank of laser diodes, a source of pump light or any other suitable pumping mechanism. Pump source 18 is controlled by a pump control 20 to pump lasing medium 16 continuously or nearly-continuously and to thus achieve a population inversion among atoms 30 of medium 16. In other words, pump source 18 ensures that there is a large number of xe2x80x9cpumpedxe2x80x9d atoms 30A indicated by full circles (i.e., atoms 30A are in an upper energy state). Atoms 30A are ready to emit light 28 when stimulated. Laser 10 has a cavity 22 defined between a high reflector 24 and an output coupler 26.
When Q-switch 12 is in the on state it prevents light 28 emitted by atoms 30A of lasing medium 16 from setting up resonant modes between mirrors 24 and 26 of cavity 22 (e.g., by deflecting light 28 out of cavity 22). Hence, loss in cavity 22 is high and no output light 28 is coupled out through output coupler 26. As Q-switch 12 is turned off, the loss in cavity 22 decreases and once it equals the gain (first intersection), stimulated emission takes place, as shown in FIG. 2A. More specifically, as loss xcex3(t) drops below gain g(t) laser 10 starts to build up and light 28 is out-coupled through output coupler 26 (see FIG. 1) in the form of a pulse 32. The peak of pulse 32 generally coincides with the time at which gain g(t) and loss xcex3(t) are once again equal (second intersection). After that, pulse 32 decays along with decreasing gain g(t).
Typically, Q-switched laser 10 is operated to produce a number of pulses 32 at a certain repetition rate, as shown in FIG. 2B. This repetition rate is shown as fixed, but it may also vary with time. For that purpose, pump source 18 is set up to continuously pump medium 16 at a constant pump rate Rp. Meanwhile, loss xcex3(t) is periodically modulated by Q-switch control 14, which opens and closes Q-switch 12 very rapidly. Thus, loss xcex3(t) changes between a low level (Q-switch 12 off) and a high level (Q-switch 12 on). In response, lasing medium 16 generates photons xcfx86(t) of light 28 in pulses 32, as shown. The population of atoms 30A in the upper state is at a high or initial level Ni before each pulse 32. A number of photons xcfx86(t) of light 28 are emitted as a function of time from atoms 30A during pulse 32. The population of atoms 30A in the upper state reaches a low or final level Nf after each pulse 32. Once pulse 32 is completely out-coupled from cavity 22, Q-switch control 14 waits and then turns Q-switch 12 back on to build up the population of atoms 30A to the initial level Ni in preparation for subsequent pulse 32.
After each pulse 32 gain g(t) is depleted well below the lasing threshold and remains there for a substantial amount of time even while being pumped by pump source 18 in preparation for subsequent pulse 32. In fact, when laser 10 is continuously pumped at rate Rp, as shown in FIG. 2B, gain g(t) is below threshold without the aid of Q-switch 12 being turned on for a duration after pulse 32 that is significant. At high repetition rates this duration is a substantial percentage (5 to 50%) of the interpulse time xcfx84p. When laser 10 operates at low repetition rates this duration is a substantial percentage (5 to 50%) of the lasing medium""s 16 fluorescence lifetime (xcfx84) (the lifetime of atoms 30A in the upper state).
Given this situation, the prior art teaches that Q-switch 12 should be turned on after all the useful energy of pulse 32 is extracted from cavity 22, which sets a minimum time, but before laser 10 reaches the lasing threshold and again emits, which sets a maximum time. Avoiding this later emission ensures that no energy is taken away from the desired subsequent pulse 32. Consequently, the exact time when Q-switch 12 is turned back on after pulse 32 can be any time before laser 10 reaches the lasing threshold. In practice, it does not matter if this time is longer or shorter, as long as it is neither too short, so it does not interfere with the out-coupling of pulse 32, nor too long, so it does not fail to store energy for next pulse 32. Thus, Q-switch 12 is set to turn on after a xe2x80x9csafexe2x80x9d intermediate time to ensure stable operation. The prior art also notes, that setting Q-switch 12 to be turned on right after pulse 32 produces instabilities in power levels of subsequent pulses 32, fluctuations in build-up times, as well as artifacts (e.g., secondary emissions). For further theory of operating Q-switched lasers the reader is referred to William G. Wagner et al., xe2x80x9cEvolution of a Giant Laser Pulsexe2x80x9d, Journal of Applied Physics, Vol. 34, No. 7, 1963, pp. 2040-5 as well as Walter Koechner, xe2x80x9cLaser Engineeringxe2x80x9d, Springer Series in Optical Sciences, Vol. 1, Springer-Verlag, Berlin Heidelberg, 4th edition (1996), Chapter 8, and Orazio Svelto, op. cit.
Due to the above-mentioned intricacies as well as other considerations, most Q-switched lasers are operated at their fundamental frequency within the xe2x80x9csafexe2x80x9d regime. Thus, for reasons that will be explained by the invention, most Q-switched lasers are not fully optimized for practical applications where intracavity frequency conversion is required. In other words, most Q-switched lasers are not well-adapted to have frequency conversion elements (e.g., nonlinear optical materials for frequency doubling) positioned inside the laser cavity for converting the fundamental frequency to another desired frequency.
It would be an advance in the art to improve the efficiency of stable Q-switched lasers which take advantage of intracavity frequency conversion.
In view of the above, it is a primary object of the present invention to provide a method for operating Q-switched lasers with intracavity frequency conversion. Specifically, it is an object of the invention to provide a method for timing the turning on and turning off of the Q-switch to ensure efficient and high power operation of such intracavity frequency converted lasers.
It is another object of the invention to provide effective guidelines for determining the timing for turning the Q-switch on and off.
Yet another object of the invention is to provide a Q-switched laser with intracavity frequency conversion adapted for open and closed-loop operation.
These and numerous other advantages of the present invention will become apparent upon reading the detailed description.
The present invention provides a method for operating a laser which has a Q-switch and also performs intracavity frequency conversion. The intracavity conversion is performed by one or more intracavity frequency conversion elements provided in the laser cavity for converting the fundamental frequency of the laser to a desired, converted frequency. The method calls for turning off the Q-switch to deplete a gain of the laser and thereby generate a primary pulse at the fundamental frequency and, through frequency conversion of the primary pulse in the intracavity frequency conversion element or elements, a secondary pulse at the converted frequency. The Q-switch is then turned back on before the gain is fully depleted in the generation of the primary pulse. In particular, the Q-switch is turned back on such that a certain amount of energy of the primary pulse is retained in the laser. Preferably, the amount of energy of the primary pulse which is retained in the laser, i.e., is not out-coupled from the laser, is at least 1% of the primary pulse. At the same time, it is preferable that the Q-switch be turned on after a majority of the secondary pulse is out-coupled from the laser. This majority is preferably selected or adjusted such that the laser retains a certain pulse-to-pulse stability in said secondary pulse. In other words, a subsequent secondary pulse following the secondary pulse being out-coupled should exhibit substantially the same parameters, e.g., peak power or energy as its predecessor.
The Q-switched laser is well-suited for use at pulse repetition rates larger than 1/xcfx84, where xcfx84 is an upper state lifetime of the laser. Specifically, a Nd:YAG laser, as an example, can be operated at repetition rates of 10 kHz and higher, e.g., 30 kHz and higher.
The method of the invention can be applied in conjunction with various types of frequency conversion elements. For example, the frequency conversion element can be a nonlinear optical material which performs a frequency mixing operation. Such operations can include second harmonic generation, third harmonic generation, fourth and higher harmonic generation, difference frequency generation, sum frequency generation, parametric amplification and parametric generation.
In some embodiments the laser can be pumped continuously (cw pumping) at a certain pump rate Rp. In other embodiments the laser can be pumped nearly continuously.
In yet another embodiment of the method, the Q-switch of the laser with intracavity frequency conversion is turned back on after a majority of the secondary pulse is out-coupled from the laser and a certain portion of the primary pulse is retained in the laser. Again, this majority is preferably selected or adjusted such that the laser retains a certain pulse-to-pulse stability in said secondary pulse. Meanwhile, at least 1% of the primary pulse should be retained.
In still another embodiment of the method, the Q-switch is turned on and off repetitively to generate a train of primary and secondary pulses at the fundamental and converted frequencies respectively. After a majority of the secondary pulse is out-coupled, the Q-switch is turned back on for a majority of the train""s interpulse time xcfx84p, which is the time between subsequent pairs of primary and secondary pulses. In particular, the Q-switch is turned on for more than 95% of the interpulse time xcfx84p. In addition, the Q-switch is also adjusted so that a certain portion, e.g., at least 1% of each of the primary pulses is retained in the laser.
A Q-switched laser with intracavity frequency conversion in accordance with the invention is equipped with the appropriate Q-switch, such as an active Q-switch, and a control for turning the Q-switch on and off. The laser also has a monitoring unit for monitoring the power of secondary pulses at the converted frequency. The Q-switch control is set for turning off and turning on the Q-switch such that the power of the secondary pulses is maximized. This can be accomplished by providing a closed loop feedback from the monitoring unit to the Q-switch control. Alternatively, the adjustment can be performed in open loop operation, e.g., during an initial calibration of the laser.
The laser can be a cw pumped or nearly cw pumped laser and can be equipped with any suitable active Q-switch. For example, the Q-switch can be selected from among acousto-optic Q-switches and electro-optic Q-switches.
A detailed description of the invention and the preferred and alternative embodiments is presented below in reference to the attached drawing figures.